The need for bandwidth and reach in optical transmission has given rise to a need to use more advanced modulation formats. While, at the present, amplitude modulation is still the predominant format, advanced modulation formats often make use of phase modulation. Among the advanced modulation formats that are likely to be implemented first, owing to their simplicity, are optical duobinary (ODB), differential phase shift keying (DPSK), and differential quadrature phase shift keying (DQPSK).
For some modulation formats (such as ODB, DPSK and DQPSK), a logical exclusive-OR (or modulo 2 addition) is necessary in the modulator or demodulator. As the implementation of such a device in the electrical domain gives rise to difficulties, it has been proposed to implement such a function in the optical domain using an optical delay line interferometer (DLI).
An optical DLI is a simple device in principle. An incoming optical signal is split into two paths. The signal in one path is delayed by a time corresponding to one bit and the signals in the two paths are coherently re-combined. Generally, the splitting and the re-combination are each performed in a respective optical 3dB coupler, in which case the DLI has two outputs, corresponding to the coherent sum and difference respectively of the optical signal and the delayed optical signal. Thus, if the optical signal and the delayed optical signal are in phase, the sum output will be comparable in magnitude with the original optical signal whereas the difference output will be approximately zero, whereas if the signals are π radians out of phase the difference output will be comparable in magnitude with the original optical signal whereas the sum output will be approximately zero. If the DPSK signal is coded so that a phase change of π radians corresponds to a digital ‘1’ and a zero phase change corresponds to a digital ‘0’ the sum output of the DLI, when the DLI is correctly aligned, is an ODB optical signal corresponding to the complement of the data. The detected outputs are applied to respective inputs of a differential amplifier to obtain the received data signal. Thus, an optical DLI can, in principle, act as a decoder for optical DPSK signals, or it can be used to produce ODB signals.
Similarly, a decoder for optical DQPSK signals can, in principle, be constructed using two DLIs, of which one has a delay of one symbol in the delayed path, as for the DPSK detector, and the other has a delay of one symbol plus a phase shift of πE/2 radians.
Since interferometers rely for their operation on the constructive/destructive interference between two optical fields, a DLI is particularly sensitive to the setting of the delay. Since the delay is dependent on temperature, laser frequency variation, polarization state etc. and must be set accurately, it is necessary in a commercial system to employ an automatic control.
The normal methods of providing automatic control in receiver equipment do not readily transfer to the control of the delay in a DLI, or else they involve substantial extra expense.
E. Swanson et al., ‘High Sensitivity Optically Preamplified Direct Detection DPSK Receiver with Active Delay-Line Stabilization’, IEEE Photonics Technology Letters, vol. 6, pp. 263-265, Feb. 1994, addresses an automatic control for the stabilization of the optical delay line interferometer, but this solution requires the carrier to be present in the transmitted spectrum. This requires a degree of deliberate misalignment of the phase modulation, in that instead of the phase difference between consecutive bits being 0 or π, it is 0 or slightly less than π, which results in a degraded signal or additional penalty.
German patent application No. 10 349 736.6 proposes using the RF-power after the differential amplifier as feedback signal for a control loop. Although this is a highly effective method, it involves additional RF detection circuitry that adds to the overall costs.
K. Sticht et al., ‘Adaptation of electronic PMD equaliser based on BER estimation derived from FEC decoder’, in Proc. ECOC'01, Paper WeP39, Amsterdam, 2001, disclose controlling a polarization mode dispersion equalizer, and also the sampling phase in an amplitude-modulation optical receiver using the bit error rate (BER) as determined by a forward error correction (FEC) decoder as a feedback signal. This is a very cost-effective method since, in practice, receivers need to incorporate FEC decoding in any case, so this does not represent significant extra cost, as it only involves slight modification or addition to existing apparatus, needed for one purpose, to adapt it to a second use. It would, in principle, be possible to apply this to the control of the delay in a DLI in an optical receiver. Although FEC control can be implemented easily, however, there is a problem, since the delay value is just one of several variables that need to be controlled, and all of them affect the BER, which is precisely why they do need to be controlled. This means that optimization of BER involves a number n of control variables, (n>1). This is an especially severe problem during startup, since an n-dimensional space has to be covered to find the optimum bit error rate, which slows down the process.